First plant group: non-vascular bryophytes (mosses)

  • Mosses, hornworts and liverworts
    • Fossils of bryophyte spores ~470mya


  • Non-vascular; ground hugging carpets
    • bodies to thin to support height growth


  • Have a rhizoid but not a root
    • anchors plant
    • does not uptake water
    • need to live near water


  • Gametophyte dominated life cycle
    • sperm need water to swim to egg on gametophyte

Vascular plants were the next evolutionary step…Why?

Vascular tissues: tissues for conducting nutrients and fluid internally

The big picture: Vascular plants now dominate Earth


  • Bryophytes ruled for <100 million years
    • vascular plants show up ~425mya


  • Plants then evolved vascular tissues
    • light competition selection pressures
    • tissues connected leaves to roots


  • First vascular plants lacked seeds
    • sperm still needed water to swim
    • lycophytes and ferns = 1st groups


  • Fern and lycophyte diversity now diminished

Ancient seedless forests now power our planet: Coal!

Vascular system: the basics (add to phylogeny)


Xylem = water conducting cells (one way)
Phloem = sugar sap conducting cells (multi-directional)


  • Conducting cells also provide biomechanical support
    • lignin in cell walls
    • does not rot easily
    • does not collapse easily
    • ‘think’ bundles of rigid pipes
    • allows plants to grow vertically


  • Vascular tissues move liquid under tension


  • Vascular tissues evolved independently several times

Evolution of roots (add to phylogeny)

  • Vascular tissues belowground
    • extension of ancient stems
    • bryophytes have no ‘true’ roots


  • Absorb water and nutrients
    • active explorers of soil
    • most elements for plant growth are in soil
    • often in symbiosis with fungi


  • Anchor the plant in place
    • support vertical growth


  • Fossils show roots in lycophytes ~ 400mya
    • convergent evolution in ferns

Evolution of leaves (add to phylogeny)


  • Photosynthetic organ on plants
    • shape generally maximizes surface area exposed to sunlight


  • Microphylls: leaves with single unbranched vein
    • vein: leaf structure containing xylem + phloem
    • unique to lycophytes (add to phylogeny)


  • Megaphylls: leaves with many branched veins
    • majority of vascular plants (add to phylogeny)


  • Why do more veins matter?

Evolution of leaves: Sporangium

  • Sporophyll: modified leaf with sporangium
    • vastly different looks
    • fertile leaf (w/ spores)


  • Sporangium: spore container (house)
    • ferns: clusters of sporagnia called ‘sori’
    • lycophytes: ‘stobilus’ holds many sporagnia (like a cone)
    • angiosperms: structures inside of flowers


  • Spore mother cell: cell inside sporagnium
    • diplod cell (2N) undergoes meiosis to make a spore (1N)


  • What is the ploidy of each cell type in this picture?

Seedless vascular plants have free living separate generations


  • Sporophyte: The fern you see in the woods
    • now the dominant stage of the lifecycle


  • Gametophye: independent haploid generation
    • start out female or bisexual


  • Dependent embryo (2N) develops and replaces gametophyte
    • in moss, sporophyte develops and dies on the living gametophyte


  • What does natural selection favor diploid dominant lifecycles?

Homosphorous life cycle (mostly) with dominant sporophytes

Sporophytes are larger and long lived, gametophytes are smaller and shorter lived, each spore produces a bisexual gametophyte

Lycophytes: First vascular plants



  • Independent, branched sporophyte


  • Lignified vascular tissue, tracheids & roots
    • tracheid = special xylem cell


  • Leaves are microphylls
    • fertile leaves have strobili (spore house)


  • Some with heterospory (Selaginella spp)
    • male (micro-) and female (mega-) spores
    • male (micro-) and female (mega-) gametophytes

Ferns


  • Whisk Ferns, Equisetum & Ferns
    • mostly homosporous


  • Homospory = spores that germinate to produce bisexual (both male and female) gametophytes


  • Leaves are mostly megaphylls
    • whisk ferns lost leaves
    • Equistem has reduced megaphylls (tiny leaves)

Ferns


  • Whisk Ferns, Equisetum & Ferns
    • mostly homosporous
    • whisk ferns do not have roots


  • Leaves are mostly megaphylls
    • whisk ferns lost leaves
    • Equistem has reduced megaphylls (tiny leaves)


  • New Gaga genus with 19 species
    • “We wanted to name this genus for Lady Gaga because of her fervent defense of equality and individual expression” Pryer @ Duke University

The consequences of homospory




  • One spore → one gametophyte → bisexual gametophyte
    • everything is haploid (1N)


  • Produces both sperm and egg via mitosis


  • A lot of self fertilization
    • What is good/bad about this?

How ferns avoid self-fertilization…


  • Fastest developing gametophyte (female) releases chemical hormone


  • Hormone turns other developing gametophytes all male
    • sperm from everywhere!
    • female power - makes her own boyfriends


  • Archegonia (F) and Antheridia (M) can develop at different times
    • if fertilization doesnt happen, female gametophyte becomes bisexual